Machine control apparatus using wire capacitance sensor

ABSTRACT

A system for controlling the operating speed of a can machine having an input conveyor for temporarily storing cans while moving them along a path toward the machine includes a metal wire capacitance sensor. The wire may be arranged either as a linear sensor if it is desired to determine the number of cans in a single file queue; or it may be formed in a repeating pattern and serve as an area mass sensor. The capacitance formed by the sensor wire and the can is connected in a capacitance bridge circuit which is excited by an oscillator. A detector circuit coupled to the bridge circuit generates an output signal representative of the number of cans in the conveyor. Output drive circuitry including an optical coupler for isolating the detection circuitry from the machine under control, transmits a signal for a desired control function to a remotely located variable speed drive controller for controlling either the machine to which the cans are being fed or a machine which is feeding the cans to keep the size of the can queue at a desired value.

BACKGROUND OF THE INVENTION

The present invention relates to a control system for controlling theoperating speed of a can machine. As used herein, a "can machine" is amachine for receiving cans or empty can shells without tops for fillingthe cans, applying a top and sealing it, or labeling the can. In anautomated can production line, a number of machines are arranged toperform sequential operations on the cans which are normally conveyedfrom one machine to another along a conveyor. If the upstream machine isprocessing cans faster than a downstream machine is capable ofprocessing the cans, there will be a net build-up of cans in theconveyor. Similarly, if the downstream machine processes cans fasterthan an upstream machine is capable of producing them, the downstreammachine will quickly begin to have "misses", thereby reducingefficiency.

It has long been a practice to sense cans in a queue, such as a singlefile queue and to speed up the downstream machine or slow down theupstream machine if the cans are accumulating in number in the queue or,conversely, to slow down the downstream machine or speed up the upstreammachine if the number of cans in the queue is diminishing.

The sensing may be done mechanically, with feelers and limit switches,or it may be done electrically. A typical electrical system for sensingthe number of cans in a single file queue is to provide a plurality ofproximity sensors along the input conveyor in which the queue is formed.For example, proximity sensors may be located at six inches, twelveinches, eighteen inches, and twenty-four inches from the input of amachine. If there are enough cans only to fill the conveyor up to thenearest sensor, then only that sensor generates a signal to controllogic circuitry for performing a speed control function. If the twonearest sensors have cans adjacent them, then separate signals aretransmitted to the controller for performing a speed control function,and so on. This type of sensing is acceptable, in some cases, but itdoes not provide a single signal representative of the total number ofcans in the queue. If such a signal were provided, it would have theadvantage of permitting a more continuous range of control of thevariable speed controllers for the machines. Such variable speedcontrollers are well-known and capable of operating in response to muchsmaller increments of signal change than are permitted by the discretesensors mentioned above.

Moreover, in the case of a conveyor having room to permit more than asingle file of cans, the problems of sensing the total mass of canspresent and providing finer control increments is even more difficultwith existing commercial techniques.

SUMMARY OF THE PRESENT INVENTION

According to the present invention, in the case of a single fileconveyor queue, a sensor wire is extended along the conveyor in thedirection of travel of the cans, preferable on both sides of the cans.The sensor wire is mounted by means of insulating spacers or guides at afixed distance from the metal frame of the conveyor. With no canspresent, the wire and frame form a fixed capacitance. As the number ofcans adjacent the wire increases, the effective capacitance between thewire and frame/cans also increases. This is referred to as the "input"capacitance. The value of the input capacitance is measured by acapacitance bridge circuit which is excited by an oscillator. An outputsignal is generated representative of the number of cans in the conveyorqueue. It will be appreciated that this signal is not representative ofa discrete number of cans, but, in effect, is an analog signalrepresentative of the input capacitance which, in turn, is a function ofthe total surface area of cans present in the conveyor relative to thelocation of the sensor wire. Theoretically, the output signal thus is asignal representative of the total number of cans fully within thesensing range of the input wire and any fraction of a can enteringand/or leaving the conveyor adjacent the sensor wire.

By thus generating a signal which is a more accurate analog of the totaleffective area of cans in the queue, the variable speed controller(whether it is associated with a downstream machine or an upstreammachine) may be more effectively controlled so as to reduce large swingsor variations in the number of cans in the queue, thereby providing asmoother control function.

Other features and advantages of the present invention will be apparentto persons skilled in the art from the following detailed description ofa preferred embodiment accompanied by the attached drawing whereinidentical reference numerals will refer to like parts in the variousviews.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an overall diagrammatic view of a control system for a canmachine incorporating the present invention;

FIG. 2 is a view, partly in diagrammatic form, showing a single fileconveyor incorporating the present invention with the various componentsshown in perspective view;

FIG. 3 is a top fragmentary view of the central portion of the singlefile conveyor of FIG. 2;

FIG. 4 is an end view of the conveyor of FIG. 2;

FIG. 5 is a circuit schematic diagram of the detecting circuitry of FIG.1;

FIG. 6 is a perspective diagrammatic view of a conveyor using an areamass sensor;

FIG. 7 is a perspective diagrammatic view of an alternate form of areamass sensor for a conveyor;

FIG. 8 is a perspective view of an area mass sensor; and

FIG. 9 is a bottom view of the area mass sensor of FIG. 8.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first to FIG. 1, a can machine is diagrammatically representedat 10. The can machine 10 may be of conventional design; and, as iswell-known in the art, it may be used to perform any one of a number ofprocesses on incoming cans such as those designated at 11. Suchprocesses may include filling, sealing, labeling or the like. The canmachine 10 is driven by a motor 12 which, in turn, is controlled by avariable speed drive 13, which is also of conventional design.

The cans 11, as illustrated, are of conventional shape, with cylindricalwalls and flat tops and bottoms. The cans 11 are fed along a single-filetrack conveyor generally designated 15 which, in the illustratedembodiment, receives the cans at an inlet area generally designated 16with the axes defined by the cylindrical side walls of the cansextending in a generally vertical direction. The conveyor 15 is formedsuch that the cans are gravity fed to the can machine 10 and twisted intransit such that their axes extend horizontally at the input of the canmachine 10. This is not the only application, i.e. machines may be fedby other means such as air conveyors, cables driven conveyors, etc.

The conveyor 15 is driven by a motor 17 which, in the illustratedembodiment, is controlled by a variable speed drive 18.

While not shown in FIG. 1 for clarity, the conveyor 15, whether it is abelt type conveyor or an air conveyor, has a metal frame. In the priorart method of detecting the quantity of cans in the single file queue onconveyor 15 described above, a number of proximity sensors comprisingprimarily a metal plate forming a static capacitor with the frame of themachine, were placed at predetermined distances along the conveyor 15.When a can or a number of cans were near a given sensor, the capacityassociated with that sensor increased, and by placing discrete proximitysensors along the length of the conveyor, and providing circuitry forsensing the capacitance at each proximity sensor, a determination wasmade in electrical circuitry as to the length of the queue.

According to the present invention, a single sensing capacitor providingan input capacitance is formed. In the embodiment illustrated in FIG. 1,the sensor wire is generally designated 20. It extends from a connectorblock 19 adjacent the inlet of the can machine 10 along one side of theconveyor at the approximate mid-point of the axial length (i.e., theheight) of the can. The wire is turned upwardly to form a verticalsection 21 and crosses over the top of the cans by means of a transversesection 22 and is then turned downwardly again at 23 and extends alongthe other side of the line of cans as partially seen at 24. The portion24 of the sensing wire 20 extends along the other side of the cansparallel to the segment 20 and terminates just short of the inlet of thecan machine 10. If the in feed is long enough to generate a sufficientlystrong signal, the wire 20 may be strung on one side only.

The wire 20 is connected at terminal block 19 to a conductor 26 whichpreferably may be the center conductor of a coaxial cable which couplesthe wire 20 electrically to the input of a detector circuitdiagrammatically represented by the block 27. The detector circuit 27includes a capacitance bridge which will be described in greater detailbelow, and generates an output signal on a line 28 which isrepresentative of the value of the capacitance sensed on the wire 20,and thus representative of the number of cans in the line on conveyor15. The output signal of the detector circuit 27 may be a low frequencysignal which is fed to a remotely located control circuit 30.

The control circuit 30 may be of conventional design. The controlcircuit 30 generates a control signal depending upon the application. Ifit is desired to control an upstream machine such as the conveyor motor17, then the control signal from the control circuit 30 generates asignal to variable speed drive 18 which will slow down the motor 17 asthe signal from the detector circuit 28 indicates an increasing numberof cans on conveyor 15. Conversely, if it is desired to control the canmachine 10, then the control circuit 30 generates a signal to thevariable speed controller 13 which causes the motor 12 to increase inspeed as a greater number of cans is sensed in the conveyor 15.

The control circuit 30 and the machine being controlled by it may belocated a substantial distance from the detector circuit 27 whichpreferably is located within a few inches of the end of the sensor wire20. Thus, there can exist a substantial difference in ground potentialbetween the earth ground of the frame of conveyor 15 and the earthground of the machine being controlled. This difference in groundpotential is exacerbate considering the nature of the environment inwhich the machines work. That is, there normally is a substantial amountof electrical "noise" in and around the machinery of a can factorybecause of the motors, controls, etc. which are used. Any such "noise"or difference in ground potential can greatly effect the sensitivity ofthe detector circuit 27 and care must be taken to protect againstdifferences in ground potential, as will be described.

Turning now to FIG. 2, there is shown a short section of conveyor trackgenerally designated by reference numeral 33. The cans shown in FIG. 2may be thought of as moving from left to right along the conveyorsection 33.

The conveyor section 33 shown in FIG. 2 is shortened from that whichwould occur in a practical application for purposes of brevity. Spacedalong the conveyor section 33 are rectangular brackets 34, 35 made ofbar stock and sized so as to permit the cans 36 to move between them onthe conveyor belt 38.

A pair of wire guide rails 39, 40 extend between the brackets 34, 35 formounting and spacing the sensor wire which again generally designated byreference numeral 20. A crossover assembly generally designated 41 ismounted to the top of the bracket 35 for supporting the wire as itcrosses from one side of the conveyor to the other. The wire 20 ismounted on the inside of the rails 39, 41 adjacent the cans 36, and itis supported and spaced from the rails by insulating spacers such asthose designated 43 in FIG. 3.

The cans 36 are prevented from contacting the sensor wire 20 by means ofupper and lower guide rails which are designated 45 in FIG. 4 but whichare not shown in FIG. 2 for clarity of the other details.

The far end of the sensor wire 20 is connected to a connector block 19which, as described, is coupled to the detector circuit 27 by means of ashort length of conductor 26, which preferably is coaxial cable.

Turning now to FIG. 5, there is shown a circuit schematic diagram,partly in functional block form, of a detector circuit for generating asignal representative of the number of cans in the queue on conveyor 33.Again, the sensor wire is generally designated 20, and it is connectedvia terminal block 19 to form one leg of a capacitance bridge generallydesignated 49. The capacitance between the sensor wire and the framework of the conveyor (diagrammatically represented by reference numeral50) is represented by the dashed capacitance 51 which is referred to asthe "input" capacitance. When cans are present, the input capacitanceincreases in value proportional to the number of cans which are present.

The reference capacitance leg of the bridge circuit 49 is formed from afixed capacitance 52 and a variable capacitance 53. The other twobranches of the bridge circuit 49 are formed by the secondary windings55, 56 of an isolating transformer 57. The primary winding 58 of thetransformer 57 is fed by a conventional oscillator circuit 59 which, inturn, receives power from a voltage regulator 60 which is fed by aconventional full wave bridge rectifier circuit 61. The bridge rectifier61, in turn, is energized by a transformer 62 which is driven by aconventional oscillator circuit 63 fed from a DC voltage regulator 64.The DC voltage regulator 64 receives power from a conventional DC powersupply fed along the lines 65 coupled to the circuitry by means of aconnector 66. The oscillator 63, transformer 62 and full wave rectifierbridge circuit 61 form a conventional DC to DC convertor circuitenclosed within the block 67. An important advantage of the DC to DCconvertor circuit shown with transformer 62 is that it provideselectrical isolation between the framework associated with the assemblyline being monitored and the equipment being controlled, which may beremotely located at distances up to hundreds of feet. The conventionalDC power voltage along input line 62 may be received, of course, from aremote location, but the DC to DC convertor 67 achieves isolation ofearth grounds by grounding the transformer 62 to the framework of theconveyor being monitored as diagrammatically illustrated by the dashedline 70.

The output of the capacitance bridge 49 is coupled to an operationalamplifier 72 which is provided with negative feedback represented by theresistor 73 to form a current to voltage convertor.

As the number of cans in the input conveyor increases, the effectivevalue of the input capacitance 51 will also increase, therebyproportionately increasing the current flow into the input capacitance,51, circuitry. Since the current flow into the reference capacitors 52and 53, is constant, this increase in current flow will be directlycoupled to the inverting input of the operational amplifier 72, so theoutput signal is a signal which increases in amplitude proportional tothe input capacitance associated with sensing wire 20. The output signalof the operational amplifier 72 is coupled to a phase-sensitiverectifier circuit 75 which converts the signal to a DC signalproportional to the input capacitance, and thus, to the number of cansin the input conveyor. The output of the phase sensitive rectifiercircuit 75 is coupled to a voltage to frequency convertor circuit 76which, in turn, feeds a current source 77 which drives an opticalcoupler circuit generally designated 78 and including a photo diode 79and a photo-sensitive transistor 80 which, in turn, drives a currentsource 81. The optical coupler 78 provides electrical isolation betweenthe sensing circuitry and the drive signal which is coupled along lines82 via connector 66 to the machine being controlled, which may beremotely located, as mentioned.

In the illustrated embodiment, the operational amplifier 72 receives theoutput signal from the capacitor bridge, and the output signal of theoperational amplifier 72 is then rectified to a DC voltage in the phasesensitive rectifier circuit 75. Persons skilled in the art willappreciate that the output of the capacitor bridge may equally well becoupled first to a rectifier circuit such as the phase sensitiverectifier circuit 75 and the operational amplifier converting the outputsignal to a voltage level may then be connected to amplify the alreadyrectified output signal of the rectifier circuit.

In order to facilitate calibration of the detector circuitry of FIG. 5,two light emitting diodes (LEDs) designated 85 and 86 respectively areconnected in circuit with the outputs of comparators 87, 88respectively. The output of the phase sensitive rectifier circuit 75 isconnected to the normal input of comparator 88 into the inverting inputof comparator 87. The purpose of the indicating LEDs 85, 86 is toestablish that when no cans are in the input conveyor (that is, thevalue of the input capacitance 51 is at a minimum for the operatingrange), both LEDs 85, 86 will be lit. Reference voltages V1 and V2 aregenerated from a conventional voltage divider network and coupledrespectively to the normal input of comparator 87 and to the invertinginput of comparator 88. Reference voltage V1 defines a minimum for astarting voltage range (that is, with no cans in the input conveyor),and reference voltage V2 defines a maximum voltage for the start rangewith no cans in the input conveyor. By way of example, the normal rangeof DC voltage for the output of the phase-sensitive rectifier circuit 75may be from a nominal - 8 volts DC (with no cans present) to +10 voltsDC (for a conveyor fully loaded with cans). Thus, theoretically, whenthere are no cans in the input conveyor, the output of the phasesensitive rectifier circuit 75 is at -8 volts DC. In this example, thereference voltage V1 may be -8.3 volts and the reference voltage V2might be -7.7 volts.

To calibrate the system, with no cans in the input conveyor, the outputof the phase sensitive rectifier circuit 75 should nominally be -8.0volts. If it is, then the output of comparator 87 is a "low" voltage,and the LED 85 is energized to conduct. If during calibration the LED 85is not illuminated with no cans in the input conveyor, additionalcapacitance may be added in series with the input capacitance 51. Finetuning is achieved by adjusting the variable capacitor 53 in thereference leg of the capacitor bridge circuit 49 until the output signalof the phase-sensitive rectifier circuit 75 falls below referencevoltage V2 which is a voltage representative of the upper limit of therange of starting voltages. In the example given, if V2 is set at -7.7volts, and the output of the phase-sensitive rectifier circuit 75 is at-8.0 volts (the nominal value for the range set for starting), then theoutput of comparator 88 will also be "low", and the LED 86 will beilluminated. When both LEDs 85 and 86 are illuminated, it indicates thatthe detector circuit is within an acceptable starting range, and thissignal will increase proportionally as cans are fed to the inputconveyor.

Turning now to FIGS. 8 and 9, there is shown an embodiment for an areamass sensor which may be used with a detector of the type shown in FIG.5. An area mass sensor, as distinguished from the linear type of sensordescribed above, is intended to sense cans in a conveyor or buffer zonewherein the cans may accumulate over a width greater than the width of acan so that three or four or even more lines of cans may form, such asare seen in FIGS. 6 and 7. Turning specifically to FIG. 6, the sensorwire generally designated by reference numeral 100 is formed in arepeating square wave pattern starting from a termination 101 andextending in a straight line transversely across the conveyor generallydesignated 102 on which the cans 103 are transported. The cans may betraveling in either direction in FIG. 6; and they move beneath thesensor wire 100. After traversing the conveyor area, the wire is turnedlengthwise of the conveyor and formed back across the conveyor in arepeating pattern resembling a square wave or Greek key, alwaysmaintaining uniformity of spacing and width. The output end of thesensor wire 106 is connected to detector circuitry as described above.Other patterns of wire shape could be used as well, such as triangular,sawtooth or sinusoidal.

In FIG. 7, again the conveyor is designated 102 and the cans transportedby their conveyor are designated 103. In this case, the sensor wire isgenerally designated by reference numeral 108 and is again formed in asquare wave pattern except that the pattern progresses across the widthof the conveyor to a terminating end 109 which is connected to thedetector circuitry.

Turning now to FIGS. 8 and 9, a structure of an area mass sensorgenerally designated 110 is shown as including an upper metal top plate111 which is provided with a series of apertures 112. The apertures 112perform no electronic or sensing function but allow air flow through thesensor for applications in which the cans are conveyed using forced air.The apertured plate 111 is also provided with depending side flanges113, 114 to provide stiffness and for mounting purposes.

The plate 111 is connected to first and second insulating side railsextending beneath it and designated 116, 117 for structural rigidity. Atthe right side of the area mass sensor 110, there is a mounting bracket118 on which mounting studs 119 are fixed; and a similar mountingbracket 120 including mounting studs 121 is provided at the left side ofthe sensor 110. The detector circuitry may be mounted within the housing123 attached directly to the end of the sensor.

The sensor wire is designated 124, and referring specifically to FIG. 9,it extends from a knot or bead 125 which is formed adjacent a movabletensioning block 126 along a straight portion 127 and passes through afixed end block 128 where it is turned as at 129 and then routed back ina square wave pattern by passing through the tension block 126 as at130. The sensor is thus formed by repeating the pattern. The tensionblock 126 as well as the fixed end block 128 are formed of polycarbonatematerial for strength and insulation. The tensioning block 126 isprovided with first and second threaded screws 132, 133 which, whenturned inwardly of the block 126 bear against a fixed end block 134 andurge the tension block 126 toward the right in FIG. 9, thereby evenlyapplying tension to the wire 129. A wire or strap 140 is connectedbetween mounting bracket 118 and the frame work or track 50 andcompletes the ground circuit.

In the illustrated embodiment, the sensor wire 124 forms five differentparallel paths, and its terminal end is rigidly secured beneath a buttscrew indicated at 135 where it is electrically coupled to the inputstage of the detector circuit housed within the housing 123, asindicated earlier.

Having thus disclosed in detail preferred embodiments of the invention,persons skilled in the art will be able to modify certain of thestructure which has been disclosed and to substitute equivalent elementsfor those described while continuing to practice the principle of theinvention; and it is, therefore, intended that all such modificationsand substitutions be covered as they are embraced within the spirit andscope of the appended claims.

I claim:
 1. In a control system for controlling the operating speed of acan machine having an input conveyor with a metal frame for temporarilystoring a plurality of cans while moving said cans along a path towardsaid machine the improvement comprising: a substantially flexible metalwire extending along said conveyor in the direction of travel of saidcans; means for mounting said wire physically spaced and electricallyisolated from said frame to form an input capacitance with said frame,the value of said input capacitance being modified proportional to thenumber of cans in said conveyor; circuit means for forming a capacitancebridge circuit having one branch including said input capacitance;oscillator circuit means for exciting said bridge circuit; detectorcircuit means coupled to said bridge circuit means for generating anoutput signal representative of the number of cans in said conveyor; anddrive circuit means including isolation circuit means receiving saidoutput signal of said detector circuit means for transmitting a controlsignal to a remotely located variable speed drive controller for amachine to control said controller as a function of the number of cansin said conveyor.
 2. The apparatus of claim 1 wherein said metal wireextends along one side of said conveyor spaced from the cans in saidconveyor and then crosses over said conveyor and extends along the otherside of the said conveyor in a second path, wherein said first andsecond paths of said metal wire are substantially parallel to oneanother.
 3. The apparatus of claim 1 further comprising a voltageregulator circuit receiving power from a remote location for regulatingDC power; a DC to DC convertor including an isolation transformer forreceiving power from said DC voltage regulator for generating a DC powersignal coupled to said oscillator circuit means for energizing the same;and isolation circuit means connecting the metal frame of said conveyorto said isolation transformer, whereby said detector circuit means iselectrically isolated from its remote power source.
 4. The apparatus ofclaim 3 wherein said isolation circuit means of said drive circuit meansincludes an optical coupler for electrically isolating said detectorcircuit means from remote variable speed controller responsive thereto.5. The apparatus of claim 1 further comprising first and second visualindicators; a first logic circuit responsive to the output of saiddetector circuit means for energizing said first visual indicator whenthere are no cans in said conveyor and the output of said detectorcircuit means is above a first level; and second detector circuit meansfor energizing said second visual indicator when the output of saiddetector circuit means is within a predetermined range of voltagerelative to said first level, whereby when both of said visualindicators are energized and there are no cans in said conveyor, saidbridge and detector circuitry are balanced.
 6. The apparatus of claim 5and further comprising a variable capacitor in said bridge circuit foradjusting the value of the reference capacitance in said bridge circuitequal to the input capacitance with no cans in said conveyor.
 7. Theapparatus of claim 1 wherein said variable speed controller isassociated with the same can machine into which said cans are being fedand sensed, said control signal being transmitted to said variable speeddrive controller for increasing the speed of said machine as saiddetector circuit means indicates that the number of cans in saidconveyor increases.
 8. The apparatus of claim 1 wherein said variablespeed drive controller is associated with apparatus upstream of said canmachine for reducing the speed of said upstream apparatus as the numberof cans detected in said conveyor increases.
 9. A control system forcontrolling the operating speed of a can machine having an inputconveyor with a metal frame for temporarily storing a plurality of canswhile moving said cans along the path toward said machine, theimprovement comprising: first and second guide rail means locatedrespectively on opposite sides of a queue of said cans for limiting thedisplacement of said cans laterally; an elongated sensor wire; firstmounting means for mounting said sensor wire parallel to said firstguide rail means and spaced outwardly thereof such that said cans cancontact said first guide rail and not contact said sensor wire;cross-over means for mounting said sensor wire to said conveyor to crossover said conveyor while permitting said cans to flow in a normal path;and third mounting means for mounting said sensor wire along theopposite side of said conveyor and outwardly of said second guide rails;detector circuit means including a capacitance bridge incorporating theinput capacitance formed by said sensor wire and said conveyor frame forgenerating an output signal representative of the number of cans in saidconveyor; and isolation circuit means for isolating said detectorcircuit means from its source of power and for isolating an outputcontrol signal generated by said detector circuit means from acontroller receiving said control signal.